This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A009399 #17 Dec 21 2017 17:29:12
%S A009399 0,2,-20,576,-33312,3258880,-485139456,102300807168,-29028932390912,
%T A009399 10668077137133568,-4929291212351078400,2797060130323340197888,
%U A009399 -1912137417504544127975424,1550018044651811766917922816
%N A009399 Expansion of log(1+tanh(x)*x).
%F A009399 a(n) ~ -(-1)^n * sqrt(Pi) * 2^(2*n + 1) * n^(2*n - 1/2) / (r^(2*n) * exp(2*n)), where r = A069855 = 0.8603335890193797624838934241376623334118843632... is the root of the equation r * tan(r) = 1. - _Vaclav Kotesovec_, Dec 21 2017
%t A009399 With[{nn=30},Take[CoefficientList[Series[Log[1+Tanh[x]*x],{x,0,nn}],x] Range[0,nn]!,{1,-1,2}]] (* _Harvey P. Dale_, Jun 13 2016 *)
%Y A009399 Cf. A009379, A069855, A085984.
%K A009399 sign
%O A009399 0,2
%A A009399 _R. H. Hardin_
%E A009399 Extended with signs by _Olivier Gérard_, Mar 15 1997
%E A009399 Prior Mathematica program replaced by _Harvey P. Dale_, Jun 13 2016